Circuit Diagram Knowledge

The amplifiers of a LM3900N device can be conveniently used to make a mixer unit for audio purposes; the unit enables three separate audio signals to be mixed together to produce a composite output.
The audio mixer circuit shown provides this facility using only a single LM39OON device and also enables any one channel to be selected by switches. The currents passing through the resistors R4, R8 and R,, are summed in the input circuit of the fourth amplifier. lf S; is open, amplifier 1 will be driven to saturation by the current passing through R2. lt will therefore be inactive. 

Th below shown simple mixer circuit will work with two or three channels, providing excellent input isolation and exceptional frequency response, extending well over the top end of the audio spectrum. lt is usable by one or more instruments plus microphone, or with special effects, such as mixing en input with pink noise, to give surf’. The unit will give 8db gain, and since low-level signals are involved, should be housed in an aluminum box. lf a mains supply is used, the usual anti-hum precautions must be taken. lt is useful to use scaled slider potentiometers, so that effects may be recreated.